The role of hyperplasia and hypertrophy of vascular tissue in the development of increased blood pressure in rats has been studied. Genetic studies revealed that in the spontaneously hypertensive rats a single gene accounted for nearly 60% of the observed increase in blood pressure. Further examination of a number of strains of rats with varying blood pressures revealed that heart and vessel enlargement were due only in part to the increased pressure to which they are exposed, but were significantly affected by genetic factors unrelated to those regulating blood pressure. these conclusion were drawn from studies on DNA and RNA metabolism as well as physical measurements. In other types of experiments the expression of the apparent vascular hypertrophy was studied by the measurement of the rates of amino acid incorporation into various protein fractions of brain microvessels prepared biochemically. In contrast to earlier studies on mesenteric vessels, brain microvessels from spontaneously hypertensive rats primarily exhibited and increased rate of amino acid incorporation into the collagen fraction.